1. Field of the Invention
The present invention relates generally to a font memory for display. More specifically, the invention relates to a font memory for display, which is capable of changing the number of fonts to be stored in an assigned storage region when an on screen display (which will be hereinafter abbreviated to OSD) font is stored in, e.g., a read-only memory (which will be hereinafter abbreviated to ROM).
2. Discussion of the Background
In general, mono-font data are displayed by single font data since color data are separately assigned and since it is sufficient just to display only a pixel position. On other hand, a color font for OSD is displayed by three font data of red (which will be hereinafter abbreviated to R), green (which will be hereinafter abbreviated to G) and blue (which will be hereinafter abbreviated to B) so that the font can be displayed with full color by changing the position of pixels forming the font and the rate of three colors (R, G, B).
For each of mono-font and color font of the OSD font, a predetermined number of data are stored in a font ROM. Conventional OSD fonts can not change the number of fonts since the number of color fonts and the number of mono-fonts are fixed. In the OSD, there are some cases where the number of mono-fonts to be displayed is insufficient although there is an unused color font. In such cases, the number of mono-fonts can be increased by writing mono-font data, which are to be displayed, in a region of a ROM, in which font data of a color font is to be written. However, as described above, as that the amount of color font data is three times as large as the amount of mono-font data, the data storage region of the ROM is not effectively used.
In some cases of conventional font memories, they could not be provided with any assignment region segments in which mono-fonts and color fonts are to be stored, respectively. For example, discriminate bits for mono-/color fonts are stored in a VRAM for assigning a display font, and an end address for each font is stored in the VRAM. By assigning the discriminate bit and the end address, the data mapping of fonts is treated as a block of data. Such conventional font memories using RAMs can flexibly change the number of mono-fonts and color fonts. In order to achieve this, it is required to previously calculate the number of discriminant bits and the end address to grasp these data.
On the other hand, fonts displayed by OSD are stored in a storage region of a ROM which is assigned by a character code (which will be hereinafter referred to as a CRA code). However, this CRA code has not been used for efficiently storing font data via processing operations, such as an operation for changing the assigned storage region, in view of the difference in data amount between mono-fonts and color fonts, although the CRA code has been used for assigning mono-fonts and color fonts to regions of the ROM.
As described above, in some cases of conventional font memories for display, they could not be provided with any assignment region segments in which mono-fonts and color fonts are to be stored, respectively. In these cases, when a display font is stored in a ROM, three color font data are required for a single mono-font data. Therefore, a storage region in which the mono-font data is to be written is insufficient, so that there is a problem in that the number of mono-fonts capable of being stored is limited.
If the font memory comprises a VRAM, the number of mono-fonts and color fonts can be changed by assigning a discriminant bit and an end address for a mono-font and color font. In order to achieve this, the discriminant bit and the end address must be previously calculated, so that there is a problem in that the processing is complicated.